1. Field of the Invention
The present invention relates generally to an Orthogonal Frequency Division Multiple Access (OFDMA) mobile communication system, and in particular, to a method and apparatus for transmitting ACK/NACK information over a reverse link in a Time Division Duplexing (TDD)-based OFDMA system.
2. Description of the Related Art
Orthogonal Frequency Division Multiplexing (OFDM) is widely applied to digital transmission technologies such as Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB), Wireless Local Area Network (WLAN), Wireless Asynchronous Transfer Mode (WATM), etc. In particular, OFDM, which overlaps frequency spectrums, has a high frequency-efficiency and is robust against frequency selective fading and multipath fading. In addition, OFDM can reduce Inter-Symbol Interference (ISI) with use of a guard interval, and can simply design a hardware equalizer. Recently, therefore, OFDMA based on OFDM, which is suitable for high-speed data transmission in the wireless channel, is under study as a multiple access scheme for the next generation mobile communication system, mainly led by the 3rd Generation Partnership Project (3GPP), which is an asynchronous cellular mobile communication standard group, and the 3rd Generation Partnership Project 2 (3GPP2), which is a synchronous cellular mobile communication standard group.
In the OFDMA system, wireless resources can be expressed in a two-dimensional time-frequency array as shown in FIG. 1. In FIG. 1, since the horizontal axis represents a time domain and the vertical axis represents a frequency domain, one resource unit can be expressed as one time-frequency square, i.e., one square space in the frequency domain represents one subcarrier, and one space in the time domain represents one OFDM symbol. For example, 8 OFDM symbols constitute one physical frame, and the frame can be defined as a Transmission Time Interval (TTI) of a forward-link channel that is transmitted from a Base Station (BS) to a Mobile Station (MS) in the OFDMA system.
For example, in FIG. 1, a resource unit composed of 8 OFDM symbols in the time domain and 16 subcarriers in the frequency domain is defined as a ‘tile’, which is a resource allocation unit which is allocated for data transmission in the process of scheduling MSs. FIG. 1 illustrates a 32-tile 5-MHz system bandwidth.
Meanwhile, TDD, one of the schemes for distinguishing a Forward-Link (FL) and a Reverse-Link (RL), is now under study and applied to the 2.3-GHz portable Internet system such as Wireless Broadband (WiBro), and is considered even in the Ultra Mobile Broadband (UMB) system.
TDD is a scheme in which a BS and an MS use the same frequency band in transmitting data, thereby increasing frequency efficiency. Further, the BS and the MS are allocated different time slots thereby supporting bidirectional transmission. The number of time slots allocated for each link is subject to change according to the amount of data transmitted over FL and RL for a predetermined time interval, and this can be expressed as a TDD ratio. The predetermined time interval is defined herein as a sum of the number of consecutive FL time slots and the number of consecutive RL time slots. The actual data transmission is achieved in the time zone where the defined predetermined time interval is continuously repeated.
For example, in a TDD system with a TDD ratio=1:1, FL and RL are equal in the number of time slots allocated thereto for a predetermined time interval. If the FL and RL links alternately operate in the time domain, the predetermined time interval is a time corresponding to 2 time slots, so 1 FL time slot and 1 RL time slot exist in the corresponding time interval.
Even in the case where a time interval is defined as a sum of 4 consecutive FL time slots and 4 consecutive RL time slots, since FL and RL are equal in the number of time slots allocated thereto for the corresponding time interval, the system can be considered to have a TDD ratio=1:1.
FIG. 2 is a diagram illustrating a time line that is composed of 2 consecutive FL time slots and 1 consecutive RL time slot for a TDD ratio=2:1.
The horizontal axis represents a time domain, and a square with an arrow inside represents one time slot, and means one PHYsical (PHY) frame composed of, for example, 8 OFDM symbols. Frames with a top-to-bottom arrow represent time slots allocated for FL, and frames with a bottom-to-top arrow represent time slots allocated for RL. Numerals 0 to 5 express interlace indexes, and since there are 6 interlaces for FL as shown in FIG. 2, a system supporting Hybrid Automatic Repeat reQuest (HARQ) can transmit 6 new packets within a Round Trip Time (RTT), which is the time interval from the initial packet transmission time until just before a retransmission time. For example, if a BS transmits the first subpacket of a new packet through an interlace #0 (first time slot) and an MS fails in the MS's demodulation on the subpacket, the BS retransmits the second subpacket of the corresponding packet in the next interlace #0 (tenth time slot). That is, in the HARQ system, FL RTT indicating the time from the initial transmission until before the retransmission becomes 9 TTIs.
HARQ is a combined technology of an Automatic Repeat reQuest (ARQ) technology and a Forward Error Correction (FEC) technology, generally used to increase data transmission reliability and data throughput in a packet-based mobile communication system. A receiver decodes received data by performing a predetermined inverse FEC process on the received data, and then performs Cyclic Redundancy Check (CRC) check on the decoded data to determine whether there is any error in the decoded data. If there is no error as a result of the CRC check, the receiver feeds back an ACKnowledgement (ACK) to a transmitter so that the transmitter may transmit the next data packet. However, if there is any error as a result of the CRC check, the receiver feeds back a Non-ACKnowledgement (NACK) to the transmitter so that the transmitter may retransmit the previously transmitted packet.
Through the time slots allocated for RL, ACK/NACK information and Channel Quality Indication (CQI) information are transmitted, not only for the RL data, but also for the FL data. FIG. 2 shows that ACK/NACK information for FL data transmitted through the hatched interlaces #0 and #1 are transmitted through the hatched second RL time slot (sixth time slot) in the reverse direction. That is, since TDD ratio=2:1, ACK/NACK for 2 FL time slots is transmitted through 1 RL time slot. When there is a need for retransmission due to the ACK/NACK information transmitted over RL, retransmission is achieved through the next interlaces #0 and #1. As shown in FIG. 2, there is a time interval corresponding to multiple time slots between the timing at which data is transmitted through FL time slots, the timing at which ACK/NACK is received over RL, and the timing at which retransmission is made over FL, and this is given taking into account the propagation delay between a BS and an MS, and the actual processing time required for performing modulation/demodulation and encoding/decoding.
Resources allocated for ACK/NACK information for FL data can be resources implicitly mapped to physical resources used for transmitting the FL data, or can be resources explicitly indicated through Layer 1 (L1)/Layer 2 (L2) signaling or upper layer signaling. In a 5-MHz system having, for example, 32 tiles, if all of the 32 tiles are used for data transmission through an arbitrary interlace, ACK/NACK information of a maximum size of 32 bits can be transmitted at an RL time slot, and if data is transmitted through two consecutive FL interlaces in the time domain, ACK/NACK information of a total size of 64 bits is needed.
In ACK/NACK transmission, if a half-tile that, as a resource unit, considers 8 OFDM symbols in the time domain and 8 subcarriers in the frequency domain can transmit 32 bits, as in, for example, the UMB system, then the necessary ACK/NACK information, when the amount of necessary ACK/NACK information corresponds to 32 bits, can be transmitted using the 1 half-tile. If repeated transmission is performed 4 times on an ACK/NACK signal to obtain a frequency diversity gain, 4 half-tiles are used over the entire frequency band. Therefore, from the viewpoint of the amount of resources, 2 tiles are used for ACK/NACK transmission. In addition, for a Multiple-Input Multiple-Output (MIMO) system with rank=4, a total of 2*4=8 tiles are required for ACK/NACK transmission.
In a TDD system where a ratio of the number of time slots allocated for FL transmission for a predetermined time interval to the number of time slots allocated for RL transmission is 3:2, ACK/NACK information for the data transmitted through 3 FL time slots would conventionally be transmitted to a BS through 2 RL time slots. If the amount of allocated ACK/NACK information transmitted through each RL time slot is different in this way as RL transmission corresponding to multiple FL time slots is made with multiple time slots and the number of FL time slots cannot be divided by the number of RL time slots, ACK/NACK information for FL data transmission is non-uniformly distributed for each RL time slot, i.e., in the ratio-3:2 TDD system, ACK/NACK information corresponding to FL data transmission is carried on 2 RL time slots in a ratio of 2:1 or 1:2. For example, if ACK/NACK information is transmitted in a ratio of 2:1, a maximum of 32 tiles can be allocated for data transmission in every interlace. Thus, if MIMO is not considered, 32*3=96-bit ACK/NACK information would be transmitted through 2 RL time slots. Therefore, if a ratio of 2:1 is used, 64-bit ACK/NACK corresponding to 2 FL interlaces is transmitted through the first RL time slot, and 32-bit ACK/NACK corresponding to 1 FL interlace is transmitted using the second RL time slot.
In this case, compared with time slots that transmit less ACK/NACK information, RL time slots that transmit more ACK/NACK information may impose restrictions on transmission of other information due to the ACK/NACK information. As described above, in the RL time slots can be transmitted not only the ACK/NACK information but also data and CQI information. Therefore, if a large amount of ACK/NACK information is transmitted through the RL time slots, the amount of available resources is reduced, putting limitation on resource allocation for transmission of other information except for ACK/NACK in the corresponding RL time slot. In addition, in a MIMO system with rank≧2, since the amount of necessary transmission ACK/NACK information increases in proportion to a rank value, the amount of resources over which data or CQI information can be transmitted is significantly reduced, imposing significant restrictions on transmission of information other than ACK/NACK. In this case, the transmitter would use the next RL time slot for transmission of RL data or CQI other than ACK/NACK, causing an increase, especially in RTT for RL data transmission.